As represented by nylons, diamines are used in many fields as raw materials of polyamides. A polyamide is obtained by polyconderisation reaction, under heat condition, of diamine dicarboxylate having a molar ratio between a diamine and a dicarboxylic acid of 1:1. However, since, depending on the diamine as the raw material, the mole balance between the diamine and the dicarboxylic acid may change during the polymerization reaction due to a high volatility of the diamine, the diamine may be required to be added in an excess amount with respect to the dicarboxylic acid (e.g., JP 2004-75932 A). Since the diamine to be added must be a highly pure product with which physical properties of the polyamide resin will not be deteriorated, a technology to simply purify a diamine from an aqueous solution containing a diamine salt to obtain a highly pure product has been demanded.
Conventionally, for purification of a diamine from a diamine salt (e.g., diamine sulfate), a method is known wherein an alkaline substance. (e.g., sodium hydroxide) is added to the diamine salt to increase the pH, leading to release of the diamine to produce the free diamine, followed by separation of the salt (e.g., sodium sulfate) produced by the addition of the alkaline substance by an operation such as extraction, and further purification of the free diamine by distillation (see, for example, JP 2004-114 A). However, in cases where a diamine and a salt are separated from each other by an operation such as extraction, a large quantity of an organic solvent (e.g., aniline, chloroform or the like) is required, and in cases where the diamine has a low partition coefficient, the recovery of the diamine in the organic solvent layer is low, so that repeated extraction operations with the organic solvent is necessary, which is problematic. Further, since, after the extraction operation, large quantities of the organic solvent and an aqueous solution containing the organic solvent are discharged, there are problems of increase in the cost of waste disposal and increase in the environmental load.
It could therefore be helpful to attain effective removal of a diamine salt without an extraction operation upon purification of a diamine from the diamine salt, thereby providing a method for efficiently purifying a diamine suitable as a raw material for a polyamide.